Coagulation
The blood coagulation system responds to vascular injury with local production of a clot formed of fibrin mesh and activated platelets. While this process is essential for hemostasis, dysregulated coagulation can lead to blood vessel occlusion (thrombosis), precipitating life-threatening events such as myocardial infarction, stroke and venous thromboembolism. In the classical view of blood coagulation, thrombin generation and fibrin formation can be initiated by two distinct mechanisms referred to as the extrinsic and intrinsic pathways (Davie, E. W. et al. Science. 145:1310-1312, 1964; Macfarlane, R. G. Nature. 202:498-499, 1964).
The extrinsic pathway involves binding of plasma factor Vila (NM) to extravascular tissue factor (TF) at a site of vessel injury (Mackman, N. Arterioscler Thromb Vasc Biol. 24:1015-1022, 2004). The first step in the intrinsic pathway requires the surface-dependent activation of plasma factor XII (fXII) to fXIIa in a process called contact activation (Gailani, D., et al. J. Thromb. Haemost. 5:1106-1112, 2007; Schmaier, A. H. et al. J. Thromb. Haemost. 5:2323-2329, 2007). Contact activation involves two other proteins, prekallikrein (PKK) and high molecular weight kininogen (HK). HK serves as a docking molecule for PKK on the contact surface. PKK is cleaved by fXIIa to form the protease α-kallikrein, which in turn cleaves fXII to generate additional fXIIa. Collectively, fXII, PKK, and HK comprise the plasma contact system. FXIIa generated by contact activation can activate factor XI (fXI) to fXIa, triggering a series of proteolytic cleavage events that culminates in thrombin generation and fibrin clot formation.
The plasma kallikrein/kinin system that consists of the proteins factor XII, prekallikrein, and high molecular weight kininogen was first recognized as a surface-activated coagulation system arising when blood or plasma interacts with artificial surfaces (Sainz et al. Thromb. Haemost. 2007; 98:77-83). The kallikrein-kinin system is a component of the intrinsic pathway of blood coagulation along with factors XI, IX, and VIII. These proteins have subsequently been shown to have roles in fibrinolysis, thrombin-induced platelet activation, control of blood pressure, cell adhesion and angiogenesis (Schmaier et al. Curr. Opin. Hematol. 2000; 7:261-265).
Kallikrein
Plasma prekallikrein is the precursor of plasma kallikrein, which in turn liberates kinins from kininogens and also generates plasmin from plasminogen. Plasma prekallikrein is converted to plasma kallikrein by Factor 12a by the cleavage of an internal Arg-Ile peptide bond. Plasma prekallikrein, in turn, is the product of the KLKB1 gene (MacKenzie, J. A. et al. Appl. Physiol. Nutr. Metab. 35: 518-525, 2010). Plasma kallikrein works in association with Factors 11 and 12.
Disease
Thrombosis is the pathological development of blood clots, and an embolism occurs when a blood clot migrates to another part of the body and interferes with organ function. Thromboembolism may cause conditions such as deep vein thrombosis, pulmonary embolism, myocardial infarction, and stroke. Significantly, thromboembolism is a major cause of morbidity affecting over 2 million Americans every year (Adcock et al. American Journal of Clinical Pathology. 1997; 108:434-49). While most cases of thrombosis are due to acquired extrinsic problems, for example, surgery, cancer, and immobility, some cases are due to a genetic predisposition, for example, antiphospholipid syndrome and the autosomal dominant condition, Factor V Leiden (Bertina R M et al. Nature 1994; 369:64-67).
Treatment
The most commonly used anticoagulants, warfarin, heparin, and low molecular weight heparin (LMWH) all possess significant drawbacks.
Warfarin is typically used to treat patients suffering from atrial fibrillation. The drug interacts with vitamin K-dependent coagulation factors which include factors II, VII, IX, and X. Anticoagulant proteins C and S are also inhibited by warfarin. Drug therapy using warfarin is further complicated by the fact that warfarin interacts with other medications, including drugs used to treat atrial fibrillation, such as amiodarone. Because therapy with warfarin is difficult to predict, patients must be carefully monitored in order to detect any signs of anomalous bleeding.
Heparin functions by activating antithrombin which inhibits both thrombin and factor X. (Bjork I, Lindahl U. Mol Cell Biochem. 1982 48: 161-182). Treatment with heparin may cause an immunological reaction that makes platelets aggregate within blood vessels that can lead to thrombosis. This side effect is known as heparin-induced thrombocytopenia (HIT) and requires patient monitoring. Prolonged treatment with heparin may also lead to osteoporosis. LMWH can also inhibit Factor II, but to a lesser degree than unfractioned heparin (UFH). LMWH has been implicated in the development of HIT.
Thus, current approved anticoagulant agents lack predictability and specificity and, therefore, require careful patient monitoring to prevent adverse side effects, such as bleeding complications. There are currently no approved anticoagulants which target only the intrinsic or extrinsic pathway.